It is commonplace in the electronics art to use leadframes for supporting and providing external leads to electronic elements such as semiconductor die. The leadframes are of a wide variety of metals including but not limited to, copper and copper alloys. A typical prior art leadframe suitable for forming a fourteen lead a dual-in-line package is shown in simplified plan view in FIG. 1.
FIG. 2 shows the leadframe of FIG. 1 after die attach and plastic encapsulation and before plastic deflash. It is usual during encapsulation molding to have plastic fill the gaps between the leads and the leadframe dam bars, i.e., inter-lead plastic. Sometimes, when molding is less than perfect there will also be a thin plastic flash on the flat portions of the leads, i.e., over-lead plastic flash. The plastic package body, the inter-lead plastic and the overlead plastic flash are indicated by the more heavily stippled regions in FIG. 2. In addition, some of the resin which makes up the resin-filler encapsulation material may be extruded from the encapsulation onto the leads, resulting is what is called "resin bleed". Resin bleed is often colorless and very difficult to detect.
FIG. 3 shows the leadframe after the dam bars, side-rails, inter-lead and over-lead plastic flash are removed. The inter-lead plastic is typically removed during trimming of the leadframe to remove the dam bars and side-rails. The over-lead plastic flash and resin bleed is typically removed by blasting the leads with a mild abrasive such as, for example, ground apricot pits.
It is frequently desirable to spot plate part of the leadframe with a noble metal, i.e., gold, silver, platinum, palladium, iridium or high alloys thereof. Silver is particularly useful because of the ease with which metallurgical bonds may be formed thereto and its moderate cost. The spot plating covers the ends of the leads where wire bonds or tab bonds from the die are attached sometimes the die flag as well, and is usually intended to be confined to an area within the boundary of the plastic body after encapsulation. This is shown in FIG. 1 where the lightly stippled region represents the desired spot plating area and the dashed outline represents the intended location of the plastic encapsulation.
A significant problem encountered with spot plating, is that it is very difficult and sometimes impossible to prevent excess and unwanted plating material from creeping laterally outward from the desired plating area along the edges and sides of the various leads. FIG. 4 shows a partial cut-away view, much enlarged, along an inter-lead space through the leadframe of FIG. 3. The lightly stippled region in FIG. 4 shows where excess plating material, as for example silver, has crept outward along the sides of the lead during the plating process and is now exposed after encapsulation, trim and plastic deflash. Excess silver plating material is known in the art as "silver flash" or "silver bleed". This excess silver, for example, is often in the range of 0.1-8 micrometers thick, typically 1-3 micrometers thick. This excess silver or other plating metal may arise from what is known in the art as "immersion" metal, which is typically 0.1-2 micrometers thick, and "bleed" metal, which is typically 5-6 micrometers thick, or a combination thereof. This invention is concerned, among other things, with removing such excess plating metal regardless of origin and the words "metal flash" or "plating flash" are intended to include "immersion", "bleed" and other forms of excess plating metal.
While such condition is in itself harmless, when solder is applied to the finished device or reflowed, the excess spot plating material on the edges or sides of the leads in the inter-lead space can cause the formation of metal whiskers that degrade the reliability and performance of the finished device. This is highly undesirable.
In the prior art, excess noble metal flash has typically been removed by immersing the parts in cyanide reagents which dissolve the excess noble metal. However, the use of cyanide is undesirable on many grounds well known in the art. Accordingly, a continuing need exists for improved processes for removing metal plating flash and particularly for metal deflash processes which do not employ cyanide.
The problem of plating flash removal is complicated by the other conditions associated with the manufacturing process which also must be accomplished. For example, the plastic deflash process can cause minute quantities of the blasting abrasive to become embedded in the leadframe, as illustrated in FIGS. 3. and 4. This is especially troublesome with relatively soft metals such as for example OFHC copper and Olin Type 194 copper. Also, plastic deflash may be incomplete and regions of plastic molding resin from the encapsulation may remain on the leads. If these regions are very thin they may be very difficult to detect with the unaided eye.
Also, various oxides or other dielectrics or resistive materials can form on the leadframe during manufacturing. These foreign materials are generally referred to in the art as "stains" because they frequently have a colored appearance. As used herein the words "stain" or "stains" are intended to include all such oxides, dielectrics, scales or other foreign materials that form on the leadframe during manufacture. Such stains are most frequently encountered when the leadframe is heated to several hundred degrees Celsius, as for example during bonding and encapsulation. Copper containing leadframes are particularly susceptible to such problems.
The embedded blasting material, stains and any residual plastic flash must be removed or they can interfere with subsequent solder plating, coating or reflow. Thus, a need continues to exist for improved processes for performing each of these operations and there is a particular need for an improved process which can do all three, i.e., remove excess spot plate metal, embedded blasting material, residual plastic flash and stains.
Accordingly, it is an object of the present invention to provide an improved method for manufacturing electronic devices including removing excess plating flash from the leads. It is a further object to provide an improved method including removing embedded blasting material from the leads. It is a still further object of the present invention to provide an improved method including removing stains from the leads. It is a yet further object of the present invention to provide an improved method including removing residual encapsulation resin from the leads. It is a still further object of the present invention to provide an improved method that accomplishes combinations of the above.